1. Field
The present invention relates generally to a motor vehicle stability control system that provides functions such as, for example, hill hold control, hill descent control, and other stability features such as maintaining a driver's desired velocity on inclines and declines, and reducing a vehicle's kinematic energy when the vehicle loses control.
2. Introduction
Many vehicle stability controls now include electronic stability control (ESC) and roll stability control (RSC) functions. The former is intended to control a vehicle's yaw motion and certain sideslip motion to keep the vehicle on the road, whereas the latter is intended to control the vehicle's roll motion to prevent vehicle divergence.
ESC systems have recently been enhanced to include a hill hold control feature that prevents a vehicle from unintentionally rolling backward on hill starts. To do this, brake pressure can be automatically maintained or an auxiliary clutch can be engaged to prevent the vehicle from rolling backward until the driver presses the accelerator or applies a certain pressure to the accelerator. Existing hill hold control systems are disclosed, for example, in Bosch's ESPlus® product and in U.S. Pat. No. 6,616,572.
U.S. Pat. No. 6,616,572 discloses that inputs to the system for hill hold control include a grade judging means, a shift gear train detecting means, and a foot brake operation detecting means. The grade judging means is disclosed to detect signals from a gear shift position sensor, a vehicle speed sensor, an acceleration sensor, and an input and output shaft rotation speed sensor, or to make a judgment based on a signal from an incline angle sensor.
Bosch's ESPlus® product can determine hill hold conditions when a vehicle is stopped, but may be less dependable or capable when the vehicle is moving or driving, particularly on a curved and hilly road, because hill grade is indirectly estimated through, for example, the longitudinal acceleration and the vehicle's reference velocity. Such an estimation can be inaccurate when a vehicle is dynamically driven on a curved and hilly road, where the vehicle's lateral velocity and yaw rate are both high.
Typical ESC sensor clusters include a lateral accelerometer and a yaw rate sensor. A longitudinal accelerometer can be added for four-wheel-drive vehicles. Typical RSC sensor clusters include a roll rate sensor in addition to the sensors provided in ESC clusters. Therefore, two angular rate sensors and two accelerometers are provided in RSC sensor clusters.